In many lighting applications, especially in lighting application in which Light Emitting Diodes (LEDs) are used, silicone is used for optical elements, such as for example, a lens or a light guide for guiding the light to a light exit window or improve the outcoupling of light from the LED. Silicones are sufficiently stable in LED application and are able to withstand the relatively high light dosages and high temperatures of LED applications. Such optical elements can easily be manufactured from a silicone resin.
Luminescent quantum dots are a promising material for creating lighting assemblies which emit a specific color of light, such as, for example, a lighting assembly which emits white light. The luminescent quantum dots absorb a portion of light emitted by, for example, a LED and convert the light towards light of another color. Quantum dots provide a high efficiency and relatively long lifetime if the quantum dots are well spatially separated in the matrix material. Because of the advantageous characteristics of silicone, the quantum dots are preferably embedded in silicone.
However, quantum dots and similar luminescent materials (such as quantum rods or quantum tetrapods) cannot be easily dispersed in silicone. It is known that quantum dots can easily be dispersed in liquids like toluene and some acrylic mixtures without forming clusters of quantum dots—in such liquids the quantum dots will be well spatially separated. Quantum dots are very small particles and during manufacturing they need to be added as a mixture of a solvent and quantum dots to a silicone resin. Quantum dots are not compatible with silicone resins and flocculation of the quantum dots occurs when they are mixed with a silicone resin. Other terms used for flocculation are aggregation, agglomeration and clustering. If quantum dots form agglomerates in which their mutual separation is less than a few nanometers (e.g. smaller than 7 nanometers), the quantum efficiency of the quantum dots reduces and a faster degradation upon irradiation is observed. Further, the quantum dots can still, to a limited extent, be mobile within the silicone resin and flocculation may slowly continue resulting in an even lower quantum efficiency. It is believed that the reduction of the quantum efficiency is the result of the occurrence of concentration quenching due to excitation and/or charge transfer between quantum dots. Further, undesired chemical reactions between neighboring quantum dots may also cause a degradation of the aggregates of quantum dots.
Published patent application US2012/0045850 discloses a material comprising quantum dots that is better compatible with many materials, such as water. The solution of the patent application may result in a material which is more compatible with silicone. The solution of the cited patent application is to create silica particles embedding the quantum dots. As silica particles are part of many silicone formulations the resulting silica coated quantum dots can easily be mixed into silicone resins.
In the approach in US2012/0045850 water is present in the reaction mixture preparing the silica nanoparticles. As for many quantum dots the presence of water either reduces the quantum efficiency directly and/or accelerates the degradation, the light conversion can be low or decreasing too rapidly over time. It seems that the use of, for example, water in the steps of creating the silica nanoparticles results in the creation non-light-emitting quantum dots. Thus, the quantum dots embedded in the silica nanoparticles of the cited patent show a relatively small light conversion efficiency and when the silica nanoparticles of the cited patent application are used in the silicone, the light conversion efficiency can be expected to be relatively low.